In general, a magnetic tape which is a coating type magnetic recording medium is produced as follows: a magnetic sheet with a wide breadth is prepared by coating one surface of a non-magnetic substrate with a magnetic coating composition comprising a dispersion of magnetic powder, a binder and optionally other additive(s) in a solvent; drying this coating composition to form a magnetic layer thereon; coating the other surface of the non-magnetic substrate with a coating composition for a backcoat layer comprising a dispersion of a pigment such as carbon black, a binder and optionally other additive(s) in a solvent; and drying this coating composition to form a backcoat layer; and this magnetic sheet is slit into tapes with predetermined widths in a cutting step.
Recording-reproducing systems with the use of magnetic tapes are roughly classified to a helical scan type using a rotatable magnetic head and a linear serpentine type using a stationary magnetic head. The recording-reproducing systems of these types have their own advantages, respectively. However, the recording-reproducing systems of the latter type, i.e., the linear serpentine type, have come into wide practical use as the recording-reproducing systems with the use of backup tapes, because a touching condition between the magnetic head and a magnetic tape is gentle, and because restriction in the volume of a magnetic tape cartridge is mild. As such commercial products, for example, Digital Linear Tape (or DLT) and Linear Tape-Open (or LTO) have been put on the market.
Mass storage magnetic tapes for backup are indispensable to correspond to hard discs of which the storage is increasing year after year. To increase the storage capacity of a magnetic tape, it is necessary to increase its recording area, in other words, to increase the length of the magnetic tape per reel by decreasing the total thickness of the magnetic tape. In this regard, the component which occupies the largest proportion of the volume of the entire magnetic tape is a non-magnetic substrate, and therefore, it is effective to decrease the thickness of the non-magnetic substrate so as to increase the length of the magnetic tape per reel. In the above-described DLT and LTO, a polyethylene terephthalate (PET) film or a polyethylene naphthalate (PEN) film having a thickness of from 5 to 6 μm and higher strength in its lengthwise direction is used as a non-magnetic substrate. However, from the viewpoint of mass storage, it is necessary to use a non-magnetic substrate with a far thinner thickness of 4 μm or less: for example, a backup tape comprising a non-magnetic substrate with a thickness of from 2 to 3 μm is proposed (cf. JP-A-10-134337).
In the linear serpentine type magnetic recording-reproducing system, generally, a plurality of tracks are formed on the magnetic layer along the lengthwise direction. When recording or reproducing of signals is performed on the magnetic tape, the magnetic head is moved across one track from one edge to the other edge of the track in the widthwise direction. To prevent off-track of the magnetic head, it is necessary to ensure the linear running performance of the magnetic tape fed at a high speed. Therefore, a plurality of guide rollers having flange portions are provided between the magnetic tape cartridge and the take-up reel in the magnetic recording-reproducing system. However, this leads to the following disadvantage: when the cut edges of the magnetic tape frictionally touch the flange portions of the guide rollers during the feeding of the magnetic tape, decrease in the strength of the magnetic tape is unavoidable because of the thinner non-magnetic substrate, since the flexural rigidity of the magnetic tape tends to decrease in proportion to the cube of its thickness. As a result, when the cut edges of the magnetic tape frictionally touch the flange portions of the guide rollers during the feeding of the magnetic tape, the magnetic tape is apt to deform, which makes it hard to ensure the linear running performance of the magnetic tape which is passing on the guide rollers. There is another problem: when the thickness of the non-magnetic substrate decreases, the cutting property of the magnetic sheet is reduced to degrade the smoothness of the cut edges of the magnetic tape. When the magnetic tape obtained from such a magnetic sheet is fed in the magnetic recording-reproducing system described above, a part of one of the cut edges of the magnetic tape is likely to strike the flange portions of the guide rollers, with the result that the magnetic layer or the backcoat layer is damaged to cause flaking therefrom.
In a magnetic tape for use with a low-speed helical scan type magnetic recording-reproducing system, JP-B-07-114012 proposes that flaking from a magnetic layer or a backcoat layer of a magnetic tape is suppressed by cutting a magnetic sheet with a laser beam, so that the cut edges of the magnetic layer or the backcoat layer can be positioned backward inwardly, from the cut edges of the non-magnetic substrate, in order to prevent such flaking. However, the following problems still remain: even when a magnetic tape comprising a thin non-magnetic substrate and having such cut edges as described above is fed at a high speed in the linear serpentine type magnetic recording-reproducing system, flaking from the magnetic tape at the guide rollers cannot be sufficiently eliminated, and the linear running performance of the magnetic tape cannot be improved.